As used herein, the term “gemstone” is intended to cover cut stones, or rough stones which may in future be cut.
Diamonds can be graded according to characteristics such as colour and clarity. The grading of a diamond may be used in determining its value for applications such as jewellery making. In general, the market value of a polished diamond depends on its colour, cut proportions, internal clarity and weight, known as the “Four Cs”.
Grading is generally carried out by trained professionals who view diamonds under ten times magnification. Such professionals may grade a diamond by observing its colour, and by looking for blemishes on the diamond's surface and inclusions within the diamond in order to determine its clarity. Some subjectivity is inevitable when grading is carried out by human beings, and it would be desirable to be able to sort rough or polished diamonds objectively. Furthermore, it is desirable to process large numbers of stones quickly, and this is not possible if each must be examined by a human being. Similar issues arise for other gemstones.
It would therefore be desirable to provide a system to inspect gemstones such as rough diamonds in a reliable and repeatable manner. Such an inspection system may then be employed in a sorting machine where the result of an inspection determines into which category the gemstone is sorted, and supplies means to allow it to be placed into a bin, container or the like. But equally it may be used to inspect a group or parcel of diamonds and forming a characterisation or valuation of said diamond parcel without separating them. The invention may also be applied to the inspection of industrial or synthetic diamond material.
At a basic level this inspection may determine whether the object really is a diamond, perhaps using Raman scattering, but more sophisticated distinctions may be drawn based on colour, clarity, shape or other physical properties, for example.
Regardless of the application, the prior art contains numerous examples of machines designed to inspect diamonds, often in high volumes, for example at a rate of up to 15 per second. Other examples have a much lower throughput, in one extreme taking up to 5 minutes to inspect one stone. Where the throughput is low it may be practical to load the machine by hand so that the technical task of presenting the stone to the measurement device or devices is carried out by hand (often using tweezers) but for larger volumes the task of delivering and removing the gemstone from the inspection device needs to be automated if efficient production is to be carried out.
Many prior art systems employ a container such as a roller hopper which may in the simplest case drop the stone directly into the measurement device, or alternatively into a second transporter such as a vibratory track or feeder which drops a stream of diamonds or other gemstones from a single point (i.e. the end of the feeder). The advantage of the latter scheme is that there is better control of the trajectory of the diamond because its release point is known, but there are still uncertainties in the trajectory at a later time because its launch velocity is not fully controlled and there is no control over when stones are released.
These issues present significant challenges to the designer of the measurement device because it has to cope with stones arriving at random times and over a larger range of locations than is strictly necessary. This lack of precision results in a measurement device which copes with the variation either by increased complexity or by having a compromised performance.
For example, it is well known that diffraction causes the resolution of an optical system to be compromised if it is to provide a large depth of focus. There is an underlying minimum depth of focus that may be required in order to have the entire object in focus, but if there is an additional uncertainty in position caused by the delivery system the designer of the optical system must compromise the resolution to accommodate it.
The problems are further compounded when observations from a plurality of viewpoints relative to the object are required for the measurement. A plurality of measurement devices need to be concentrated around a nominal point in space where the measurement is to take place and they may well interfere with each other. In principle the devices could be separated approximately vertically along the nominal trajectory but as the object falls uncertainties in time and position increase leading to further technical problems.